Urban metabolism is “a metaphor,” says Mark Muro, policy director for the Metropolitan Policy Program at the Brookings Institution. It’s a metaphor he’s hearing more and more in discussions about urban sustainability. Like the words “sustainable,” “resilient” and really anything labeled “clean” or “green,” “metabolism” may very well be the next big buzzword in green government. 
That’s because it’s a reemerging scientific approach that helps ecologists, engineers, urban planners and politicians better understand the physical impact of cities on the environment. Urban metabolism is a way of looking at cities as living things, says Stephanie Pincetl, director of the California Center for Sustainable Communities at UCLA. Like a living organism, she says, cities require energy, materials and water to sustain citizens. Once these are used and metabolized, what the city discards is waste. 
While the notion of urban metabolism can be traced back to Karl Marx, who used the term to describe humans’ interaction with nature, today it is a framework that academics like Pincetl and others are using to account for all natural resources that a city consumes and the emissions it produces. They are, in effect, measuring the millions of gallons of water used indoors and outdoors, the millions of kilowatt-hours consumed, and the millions of tons of trash and pollution produced. This information can be used by policymakers to target energy efficiency and conservation programs. If you don’t know how much you are consuming now, the reasoning goes, how are you supposed to know what to conserve?
Urban metabolism’s reemergence has a lot to do with today’s circumstances. “It’s the era of big data. We have more data than we had before,” Pincetl says. “We are also in this era where resource limitations are being seen, yet again, as real.” With more than 80 percent of the population living in the nation’s metro areas, resource limitations are creating increasingly real and tough challenges.
That’s where urban metabolism comes in. To conserve resources, it helps policymakers focus on the impact of their actions. For example, in 2006, when former Los Angeles Mayor Antonio Villaraigosa announced the Million Trees initiative to combat air pollution, ecologists calculated that planting a million new trees would have the drawback of increasing water consumption by 5 percent. 
Similarly, Pincetl recently discovered that outdoor water use increases with wealth. In a study for the L.A. Department of Water and Power, Pincetl and her colleagues looked at water billing data to determine indoor and outdoor water use, water use by sociodemographic characteristics, water use by size of lot, and so on. “What we were able to observe was that lower-income people saved more relatively than wealthier people,” she says. As a result, the team recommended that the city install dual residential water meters -- one for indoor water use, the other for outdoor water use -- “in order to ensure that the lower income people don’t pay more for water but that the high water users, who tend to be much wealthier, do.”
Urban metabolism provides cities with a way to analyze and enhance urban sustainability. “Whether, in this case, it is changing the rules under which water agencies operate under, whether it’s changing rules about landscape, or whether it’s looking at water rights and how they are distributed,” Pincetl says, “urban metabolism is a window into many of these kinds of issues.”